In recent years, SOI technology has proved to be a particularly useful alternative to conventional CMOS technology using bulk silicon. More particularly, the so-called “floating substrate” effects in SOI technology, which are well known to those skilled in the art, and the reduction in junction capacitances are two examples of the improved performance provided by SOI technology. However, the floating substrate effects have disadvantages.
One such disadvantage is the hysteresis effect in the threshold voltage of a transistor, which is manifested by variations in time delay. That is, variations occur in the time of propagation of a signal between the input and the output of a logic cell having such transistors, e.g., an inverter.
PD-SOI technology introduces a “temporal” dependence of the delays in such a way that the same structure may have different delays from cycle to cycle when its rate is clocked by a clock signal. A method of initializing the voltage on the floating substrate is generally used in the design of SOI circuits, and error tolerances are used to take these time constraints into account. However, such an approach may result in the performance of the structure that is produced being overestimated or underestimated.
Moreover, not only do worse case delays need to be known, but the delays in the best case situations need to be known so that synchronization problems may be accounted for. Yet, both the worse and best cases are difficult to identify since process and design parameters such as current gain, input slope, charge, supply, and temperature play a key role. Furthermore, the variable nature of the threshold voltages in PD-SOI devices is such that the propagation of a given transition between the input and the output of a logic cell leads to a different delay depending on whether the cell is under static equilibrium (DC) conditions or whether a dynamic equilibrium (steady state AC) state has been reached.
Additionally, in practice it proves to be substantially impossible to characterize a logic cell by exhaustive simulations. This is because several thousands of cycles, and therefore several hours of simulation, are needed to reach dynamic equilibrium in the case of simple inverter-type cells. As such, the characterization of a much more complex cell is essentially inconceivable using this method.